The selective detection of CO and H.sub.2 gas constituents by a stannic oxide (SnO.sub.2) element has been the topic of technical papers and publications. The stannic oxide element corresponds to an n-type semiconductor oxide which exhibits changes in electrical resistivity on exposure to gases such as H.sub.2 and CO. This oxide responds primarily to H.sub.2 in the temperature range of 200.degree.-250.degree. C. when it is intimately mixed with small amounts of sintering agent such as MgO and a catalyst such as PdCl.sub.2. When small amounts of ThO.sub.2 are added to the above oxide the surface activity is shifted towards CO gas. To the latter formulation, the incorporation of small amounts of hydrophobic SiO.sub.2 to remove water from contact with the stannic oxide enhances the sensitivity of the element to CO.
Publications describing thick film sensors, which are incorporated herein by reference include:
(1) Thick-Film CO Gas Sensors, by Nitta et al., appearing in IEEE Transactions on Electron Devices, March, 1979;
(2) CO Gas Detection By ThO.sub.2 --Doped SnO.sub.2, by Nitta et al., appearing in Journal of Electronic Materials, Vol. 8, No. 5, 1979.